diff --git a/themis/importexport/sync.py b/themis/importexport/sync.py
index bb1587d..9648270 100644
--- a/themis/importexport/sync.py
+++ b/themis/importexport/sync.py
@@ -201,6 +201,8 @@ class SyncFromThemis(UtilityView):
             if related_doc_ids:
                 related_docs = []
                 for related_doc_id in related_doc_ids:
+                    if related_doc_id is None:
+                        continue
                     if hasattr(self.questions_folder, related_doc_id):
                         related_doc = getattr(self.questions_folder, related_doc_id)
                     elif hasattr(self.documents_folder, related_doc_id):